PRIOR ART
New Procedure for Making Schmidt Corrector Plates, Applied Optics, Vol. 11, No. 7, July, 1972. The Vacuum Method of Making Corrector Plates, Sky and Telescope, June, 1972. Making Corrector Plates by Schmidt's Vacuum Methods, Applied Optics, May, 1966, Vol. 5, No. 5, pages 713-715. U.S. Pat. No. 3,693,301, Lemaitre. Study of the Fabrication of Aspherical Surfaces, Sakurai & Shishido, Applied Optics, November, 1963.
There are two techniques well known in the literature for figuring Schmidt corrector plates, the first of which is often referred to as a classical approach and the second approach is referred to as the vacuum deformation technique, as evidenced, in part, by the aforementioned cited art.
The first approach, the classical approach, involves using a glass blank of high optical quality and of sufficient thickness so that one side can be worked without the glass blank slightly bending or deflecting due to work temperature or pressure during the grinding and polishing phase. The Schmidt curve is ground into the surface by rotating the blank about its center and using grinding laps which favor the areas where more glass is to be removed. It is important in this process that most of the grinding and shaping work be done with the rigid glass blank being rotated about its center on or under a grinding tool that contacts the entire surface, the grinding lap also being rigid, and this tends to keep the glass plate a perfect figure of revolution which is essential. After the shape is roughed in by courser grits, the grinding lap is cleaned thoroughly and finer grits are applied. This is repeated using progressively finer grits until the plate is ready for polishing. It is during the fine grinding stage that the figure is checked optically. The corrector is set up with the balance of the optical system with which it is to be used and tested with an optical collimater. Null testing techniques and interferometer techniques are commonly known, which allow a worker to read the errors in glass to determine the zones which need to be worked down. Using this classical approach, a high degree of skill and training are required to read the errors and properly interpret them.
The second approach, the vacuum deformation technique, is attributed to Schmidt, the original inventor of the Schmidt corrector plate. This approach consists of using a thin glass blank as a cover for a vacuum-tight chamber. When the vacuum is applied, the thin glass blank bends into the shape of a catinary curve rather than a spherical shape. While in this bent configuration, the top side of the blank is ground and polished spherical. When the vacuum is released and if the processing has been successful, there purportedly would result a perfect Schmidt corrector plate. The vacuum deformation approach implies that a thin plate can be bent sufficiently accurately and will remain sufficiently stable during the grinding procedure to result in a usable Schmidt plate. For a visual telescope, the requirements on a Schmidt plate are so demanding that the residual errors must be a small fraction of a wave length of light or accuracy approaching a millionth of an inch. If an "O" ring is used as suggested in the Sky and Telescope article, small inhomogenouities in the glass will result in non-uniform bending and therefore astigmatism or nonconcentricity about its center. This approach is barely acceptable for use with a camera and would hardly be adequate if the corrector were to be used in a visual Schmidt Cassegrain system for example.
If in using the vacuum deformation approach, the O-ring is dispensed with and a rigid ring used, the ring would have to be perfectly shaped and even then the smallest bit of dust or film at the interface between the glass and ring would still render the rsulting plate of poor quality.
In the Applied Optics, November, 1963 article, it was suggested that a thin glass blank be bent over a mold having the inverse to the Schmidt curve. The opposite side would be ground and polished flat while in this bent configuration. The article did not specify .[.as to how the glass was to be held in place or.]. as to the accuracy with which the mold should be made, nor how it would be determined that the glass was actually in conformation with the mold. The article further was devoid of a teaching of the material from which the mold was to be made. .Iadd.The article suggested the spreading of cement into grooves within the mold as well as into the contact surfaces in order to hold together plate and mold, as well as the evacuation of air between the plate and mold. There was no discussion of a method or apparatus for evacuating. .Iaddend.
A commonly used method to determine whether or not a spherical surface on glass meets its required accuracy is to make a master glass having the opposite curve. The two pieces are placed one on the other, the combination held under a light source, flat and usually monochromatic, and the reflections at the interface between the master and the work piece are observed. If they are very close to the same curve, the light from one of the surfaces will interface with that from the other and Newtonian rings or interference fringes may be observed. The character of these interference fringes is such that the worker may accurately determine the relative match between the master and the work piece.
The present invention teaches a method for making replica contour block masters for use in producing and checking for accuracy Schmidt correctors.